Ok! Lm lại:
\(\overline{a5}^2=\left(10a+5\right)^2=100a\left(a+1\right)+25\)
Áp dụng vào ta đươc:
152 = ( 10 + 5)2 = 100.1.2 + 25 = 225
252 = (2.10+5)2 = 100.2.3 + 25 = 625
752 = ( 7.10+5)2 = 100.7.8 + 25 = 5625
1152 = ( 10.11 + 5)2 = 100.12.11 + 25 = 13225
Bác ơi ko dung máy tính thế có đc tra google ko v?? haha 
Dễ ẹc!
152 = ( 10+5)2 = 100+ 100+ 25 = 225
Tương tự:
252 = 625
752 = 5625
1152 = 13225
\(\overline{a5}^2\) = \(\left(10a+5\right)^2=100a^2+100a+25\)
Ok! 
\(15^2=\left(10+5\right)^2=10^2+2.10.5+5^2=100+100+25=225\)\(25^2=\left(20+5\right)^2=20^2+2.20.5+5^2=400+200+25=625\)\(115^2=\left(100+15\right)^2=100^2+2.100.15+15^2=10000+3000+225=13225\)\(\overline{a5^2}=\left(a.10+5\right)^2=a^2.10^2+2.10a.5+5^2=100a\left(a+1\right)+25\)
152=(10+5)2=102+2.10.5+52=225
252=(20+5)2=202+2.20.5+52=625
752=(70+5)2=702+2.70.5+52=5625
1152=(100+15)2=1002+2.100.15+152=13225